Assuming you have subscribed to an $18/mo or higher plan with digital product capability (required to enable the Remotely Hosted Downloads feature), this is caused by the minimum-price requirement for Remote Product File URLs, which cannot consider the prices assigned to Variants.
To work around this, temporarily disable Variants for the product, so you can set a regular Price of at least 0.10 and enter the Remote Product File URL, then you can re-enable Variants afterwards.
